It is well known in the papermaking industries to use vacuum or suction pipe systems in dewatering. Such systems often utilize suction pipes coupled to elongated suction slots over which a felt passes causing the dewatering thereof. In systems of this type, a variety of devices exist to create the vacuum necessary for dewatering. For example, liquid ring pumps, positive displacement pumps and centrifugal exhausters or blowers. While many circumstances and operating parameters dictate what type device in this regard is best suited for a particular application, a common desire in which every type vacuum pump is selected is that it be efficiently incorporated and utilized in the system.
In this regard, generally the vacuum pumps are sized for maximum demand vacuum conditions in the suction pipe when the felt is new. The vacuum pump will normally run at its maximum speed with a new felt. As a felt fabric fills voids during its life, it becomes less permeable requiring a higher vacuum level for dewatering. However, with the decreased felt permeability and since the vacuum pump is a constant volume unit, the vacuum level will automatically increase.
Heretofore, many systems have been devised to take advantage of increasing vacuum conditions and to effect cost and energy saving as a result thereof. See i.e., U.S. Pat. Nos. 4,308,077, issued Dec. 29, 1981; 4,329,201, issued May 11, 1982; and 4,398,996, issued June 19, 1981. For example, in the variable vacuum liquid ring pump having constant flow, as set forth in U.S. Pat. No. 4,398,996, a variable speed drive motor is provided and is responsive to an increase in the vacuum level in the suction pipe. The motor is activated to slow down the vacuum pump as the felt permeability decreases, thereby retaining the desired level in the suction pipe. The lower pump speed results in lower drive horsepower and accordingly a savings in power, while retaining the desired vacuum level in the suction pipe.
While such an arrangement has proven satisfactory in certain applications, it is desired to provide for yet further efficiency and energy savings in a dewatering system, particularly one that utilizes a centrifugal exhauster or blower as compared to a positive displacement unit.
In copending U.S. application Ser. No. 575,447, filed Jan. 31, 1984, now U.S. Pat. No. 4,551,202, commonly assigned herein, there is a centrifugal exhauster which provides improved efficiency. The speed of the exhauster as the permeabilities of the felt decreases is automatically regulated. A variable speed motor drive for the exhauster of the type commonly found in the market place is utilized. The maximum speed of the motor can be limited by either the maximum current to the motor and/or maximum frequency setting. In the case of a variable frequency drive motor for example, the maximum speed and current may be automatically controlled by using a feedback loop.
In all exhauster arrangements of which the foregoing is no exception, the occurrence of surge can have a serious and often damaging effect on the system. While many different type surge controls are available, it would be desirable to have an automatic surge control that is particularly effective in the situation using a variable speed drive with a centrifugal exhauster in papermaking applications, since there is no specific signal that can be used to sense surge because the surge points change with speed.